During the development process for VR applications, it is useful to know exactly when an image is physically displayed to a user on a display of a HMD. While this is also true in regular computer display applications, for VR and particularly HMD applications this is particularly important due to the risk of inducing nausea and discomfort for the end user.
Additionally, when selecting which is the best or preferred HMD unit, or simply comparing multiple units, it is desirable to know characteristics of each unit, such as image persistence, application latency and latency difference between different eye displays, for example. High image persistence can cause motion blur like effects. Noticeable latency difference between different eye displays can also easily induce nausea and discomfort in users.
In order to evaluate or benchmark such units, some current VR devices have their own built in measurement systems. However, these systems are unique to that specific hardware and their measurements cannot easily be compared to other units or other manufacturer's products. Additionally, these units do not typically measure latency between eye displays, framerate smoothness or image persistence.
Furthermore, without hardware which is external from the display of a device it is not possible to detect dropped frames, i.e. those frames that are sent to the display device but which are not displayed. Reasons for dropped frames include that the frame was either sent or received to late and an older image is displayed instead of the intended new image. Such dropped frames create jittering in frame rates and can again cause discomfort for a user.